It has been postulated that the renin level does not have to be elevated in order to maintain high blood pressure. We are producing hypertension in rats by clipping one renal artery. After hypertension has developed, the clipped kidney is removed. These animals are considered "sensitized" to renin or another humoral factor. To see if this factor is renin, an infusion of renin will be started immediately after nephrectomy to replace the endogenous renin which the clipped kidney was releasing. Since this amount is not known, the amount of renin infused will be regulated to keep the blood pressure at pre- nephrectomized levels. If the cause of the hypertension if due mainly to the renin released by the clipped kidney, the replacement of the endogenous renin with the infusion of purified renin will originate plasma levels similar to the one found prior to nephrectomy. If so, it will tend to confirm the hypothesis that renin plays a direct role in the pathogenesis of renal hypertension. However, if the plasma renin levels are higher than pre-nephrectomy levels after the infusion, this will imply that renin does not play a direct role in the pathogenesis of hypertension and that it is only able to keep the blood pressure elevated when administered in pharmacological doses.